Air Venturi Bronco with optional target sights: Part 2

by B.B. Pelletier

Part 1

We get requests all the time for basic maintenance articles and fundamental articles about how to diagnose an airgun and make it shoot better. Often, I refer readers to blog reports I’ve done in the past, but today (and tomorrow!) is a blog report with something for almost everyone. It started out as a simple test of my Air Venturi Bronco rifle with different sights, but it blossomed in several different directions — answering many questions and raising issues about which many readers have indicated an interest in the past.

I didn’t plan on this report turning out the way it did. This special two-part report (today and tomorrow) is a serendipitous journey of airgun discovery. It began when I mounted the optional Bronco Target Sight kit on my rifle and thought I would be demonstrating just how accurate a Bronco can be. (The Bronco is also sold with the target sight kit installed.)

Heck (I thought), there’s no risk here. I’ve shot this same Bronco before and I know how accurate it is. What could possibly go wrong?

What, indeed. If you click on the link to Part 1 of this report provided above, you’ll see that the first groups I shot with the Bronco and its new target sights were anything but encouraging. If this had been a different rifle I might have been tempted to write it off as inaccurate (I said tempted — not a sure thing), but because I’ve shot this exact same Bronco several times in the past with great results, I knew it was something other than the rifle’s inherent accuracy at fault.

Back-bored muzzle
You readers guessed what the problem could be, and Mac and I conversed at the same time. Mac owns a Bronco, too, and he knows how accurate it is.

One early theory was that the new longer front sight mounting screws might be protruding into the rifled barrel and clipping the pellet just before it leaves the muzzle. I had noticed when mounting the four front sight riser plates that the screw holes are drilled deep, so I checked them and discovered they are drilled all the way through the barrel. The kit has two longer screws that are needed because of the four riser plates, so was it possible that one of those screws was hitting the pellet as it passed through the bore?


The four front sight riser plates require longer mounting screws. Was one of them touching a pellet?

But Mac told me the barrel was back-bored. The muzzle is not where you think it is, but it’s about seven inches deep inside the barrel — behind the front sight. Back-bored means that instead of being crowned at the end of the barrel, the muzzle is sunk deep inside the barrel with a deep-hole drill. Doing this protects the muzzle from damage and preserves accuracy longer. It can also restore accuracy to rifles that have been cleaned from the front instead of the breech. When cleaning rods scrape against the sides of the muzzle they wear the metal and cause accuracy loss. Mosin Nagant rifles are often found with back-bored muzzles.


The true muzzle of the barrel is located at the tip of the cleaning rod. So, the front sight screws are not interfering with pellets before they leave the barrel.

Look inside the barrel
Once I confirmed where the true muzzle was, it was obvious the front sight screws could not be interfering with the pellets before they left the muzzle. But what about after they left? Could a pellet still be touching the edge of a screw after it exited the muzzle?

This time, the answer was less exact. From what I could see with an endoscopic light down the bore, the screws were probably not protruding deeply enough to touch the pellets in flight unless the pellets were yawing wildly. And if they were yawing wildly, they were never going to be accurate anyway. So I stopped looking at that and checked the cleanliness of the barrel next.

Surprise!
The barrel had a constriction right at the muzzle! A brass bore brush passed from breech to muzzle (the true muzzle — not the end of the barrel) stopped abruptly at the muzzle. Something was constricting the barrel right at the point the pellet exited. The barrel needed to be cleaned, but this constriction was so abrupt that it felt like a large burr had been raised right at the muzzle. But the muzzle is seven inches deep inside the barrel, so that’s next to impossible!

The only solution was to clean the barrel, and I started with a clean brass bore brush. Don’t waste your time with a nylon brush. It isn’t stiff enough to remove the metal if there’s any. As long as the barrel is made of steel, like the Bronco’s barrel is, you cannot damage it with a brass brush.

After 20-30 passes the brush was meeting no resistance, so I then cleaned the barrel with Otis bore solvent until the patches came out clean. Now, the barrel was ready to perform at its best!

On to shooting
After all of this, I felt ready to shoot the rifle and expected it to do well. A quick read of Part 1 showed me that I tested it with H&N Finale Match Rifle pellets and JSB Exact RS pellets. Those were the first two pellets I shot. The range was 10 meters, and each group got 10 shots.

I would have loved to have shot two beautiful targets and ended this test right there, but that didn’t happen. Yes, the groups were somewhat smaller than those shot in part 1, but they were not good groups — not for a Bronco, anyway. The H&N Finale Match Rifle pellets grouped 10 in 1.059 inches — compared to the group in Part 1 of 1.668 inches. And the JSB Exact RS pellets grouped 10 into 0.82 inches this time –compared to 1.169 inches in Part 1. Yes, these are smaller groups, but they aren’t small enough for the Bronco at 10 meters. Something else had to happen.

I found the secret
This is where I’m going to end the report today. Tomorrow, I’m going to tell you what happened to change the outcome of the test. Yes, a different pellet was used, but that wasn’t the big news. In fact I’m convinced that I’ve found the secret to increasing accuracy with any Air Venturi Bronco — shooting any pellet!

You have a day to wonder, ponder, guess and discuss. Let’s see how smart you guys are.

What did I do that was different? A hint — I’ve done it before with similar results.

43 thoughts on “Air Venturi Bronco with optional target sights: Part 2




  1. B.B.,
    You didn’t includes pictures of the groups, so I assume they were fairly round. I’m thinking this rifle wants a firm hold. Makes no sense, really. It should be shooting lights out without any head scratching, but this makes it interesting.
    -hm


    • Hank,

      I intentionally didn’t include the groups because we have some readers who, once they see a large group, stop reading and figure the gun is inaccurate. If you had seen these groups you would have thought the same thing.

      Yes, they were fairly round, but each of them had a couple fliers that opened the group up. Tomorrow I will reveal what it took to stop those fliers from happening.

      B.B.





  2. B.B.,

    I’m very much looking forward to seeing what you came up with.

    I shoot all my springers at 25 yards. I’ve devised a system that I use to compare individual gun’s performance and my own.

    I use Shoot-n-C 6″ targets, applied to a box set on a table at the same height as my shooting bench. I shoot 30 rounds at each target, giving a maximum possible score of 300.

    All my springers are scoped, except for my Bronco Target Rifle. Using the factory-installed target sights, my best score with this rifle was 277. This is a mid-range score in comparison to my scoped springers. For comparison, my best score with a scoped springer is 285 for a Crosman Storm XT with a Tasco 3-9x40mm scope.

    As you can see, I’m no Matthew Quigley. But I do keep a notebook of each trip to the range, my score and what I shot (and what pellet was used).

    When I got my Bronco, I was concerned that there may be an interference from the front sight screws as mentioned in the first chapter of this blog. I could find no evidence of this. I have not tried cleaning the barrel. I would not expect to need to clean the barrel of a new gun.

    I am satisfied that my new Bronco can shoot with comparable accuracy to my scoped springers (better than most, actually). But if there is a secret to gaining more accuracy, I want to know.

    For what it is worth, I think the key to the accuracy of the Bronco is in its trigger. The release is so light that the gun can be fired without jarring it with excessive force on the trigger.

    As with all my spring guns, I use Tom’s Artillery Hold.

    Les


    • Les,

      Sometimes a brand-new barrel has corrosion left there by the bluing salts. That’s why it’s sometimes good to clean even a new barrel.

      B.B.


    • Les,

      IMHO new airgun barrels need cleaning immediately with a bronze or brass brush loaded with jb bore paste followed by patches followed by your favorite oil (otis bore solvent, fp10, ballistol, etc. etc.) then more patches until they come out clean. My last step is to put a patch through the barrel with your favorite oil and follow with a clean, dry patch. The alternative is to shot a few tins of pellets through your new barrel and not expect much accuracy until the first tin of pellets are shot. There are exceptions but my regiment is cleaning the barrel of any new airgun that arrives at my house. If nothing else it takes a dirty barrel out of the accuracy equation.

      ps-I’m going to guess that B.B. took his thumb off the wrist of the bronco and rested the forestock on the backs of his fingers to eliminate the flyers.

      kevin


      • I’ve put 250 rounds through the Bronco so far.

        Any point in cleaning it at this point? I have all the stuff needed to clean it (the XT and the RS2 improved after cleaning per B.B.’s instructions).

        I am satisfied with its accuracy, but am always interested in improvement.

        Or is it, “If it ain’t broke, don’t fix it”?

        Les


  3. My guess would be that you modified your hold on the rifle. Perhaps going with a heavier grip, similar to the way you shoulder a .30 .30 lever action, for example, to mitigate recoil. As I recall, this stronger hold worked for you with the gas rams like the Nitro Piston.

    Your review on the NP Trail convinced me to get the .25 NP XL . Heavy gun, heavy charge but very accurate off-hand when using this modified hold and amazingly quiet for a 30 foot pound rifle.



  4. BB,
    Finding an accuracy problem is often a long term project. My .30-06 drove me crazy when the groups went south but with none of the symptoms of a scope problem. After finding and “fixing” 3 other things that I thought were “it”, I changed the scope to one that should have been less than half as good and the groups shrunk in half from their best previous, so I wonder if it didn’t all help. My latest Keystone Cop venture was with my flintlock, where I didn’t discover the true source of its problems until I happened to see powder literally covering the pan gutter after loading and gave the touch hole liner a close look. It still wasn’t passing the 1/16″ bit I had been using to check, but it had elongated 3x along a slot (for removal), which just isn’t easy to see from all angles. Pulled it out, fitted a new one and I’m back to almost better than mediocre target shooting! Meanwhile, the barrel had been extreme-cleaned and polished bright (I can see myself in the breech plug), the sights (I made a new front one, thinking the old one was the problem) are stuck on tight enough to last until Ragnarok, etc.! In sum, I wonder if there is indeed “a secret” or if there aren’t always several things that can be checked and improved along the way and then one of them usually in conjunction with the others is the final straw, so to speak :).

    No idea what the “secret” of the Bronco is, but I’m sure you found something. I think it was an extremely important point that you made about “knowing” it was accurate before.


  5. Boy, you sure do like cliffhangers. I’ve been practicing most days with the Bronco and steadily improving. Four different pellets shot nearly the same, all far better than what you reported. Compared to my Gamo, the Bronco is relatively insensitive to the grip, so I can’t see that as a problem. The only issue I saw was that the lifted front sights can go on at an angle, so I had to very carefully level them.


    • Gerry.
      I think you unknowingly found the problem.

      The sights ARE higher on the Bronco now that BB installed the risers. Thus, the rifle is so much more sensitive to cant. I’ll bet BB put a level on the gun for tomorrow’s report.

      -Chuck


      • Maybe, but I don’t think so. It’s not that much higher and less so than some scopes, this is only 10m, and it should effect aim more than groups. It threw the sight-zero all the way to one end. Whatever it is, I’m just hoping he has something that I didn’t already do, so I can really up my target score.

        But, I think I’m seeing me and not the Bronco. My last big gain was just from increasing the lighting on the target. These eyes are getting older.




  6. BB,

    I’m still relatively new to the air-gunning world, but I’ll take a stab at this. You mentioned a flyer here or there. I’m guessing one of two things (or both). 1. Did you take greater care seating the pellet to make sure the skirt wasn’t being damaged when closing the barrel? or 2. Did you weigh the pellets so as to only use uniform weights?

    I expect there is some intricate scientific answer, but I’m a finance guy, not an engineer. That’s my best crack.

    Fun!

    SE MN Airgunner



  7. Back-bored must be the same as counterbored that I hear about on surplus rifles. I had supposed this was a liability. Now thinking about it, I can see how eliminating the rifling for some distance from the muzzle does not leave anything to interfere with the projectile. On the other hand, doesn’t a lot depend on how cleanly the rifling stops at whatever point it is counterbored to? A sloppy counterboring would seem to recreate the same irregularities as crown damage.

    I hadn’t supposed that brass brushes can remove metal but on the other hand, I’m not surprised. While cleaning, a stray bristle stabbed itself someway into my finger, just like a needle. Yeow!

    But as for the mystery, I’m completely stumped as usual. So, I will do the tarantula dance and outwait B.B. to make him tell! 🙂

    On the subject of metal wearing, I’m the proud new possessor of a barong, a sort of machete and short sword favored by tribes in the southern Philippines. So, there I am in the sweaty jungles with this amazing new weapon! Apparently, it achieved notoriety in the Spanish-American war for its ability to cut through rifle barrels. I’ve heard the same about samurai swords in WWII. Is this true? I thought the wisdom is that no blade can cut something harder than itself. I found that out the hard way (in my young and ignorant days) when I ruined my Cold Steel Voyager by using it to try to cut through a steel cable. If rifle tempering is anything close to blade tempering, I don’t see how any sword could cut through a barrel.

    But for anything short of that, such as plant and human limbs, the barong is unstoppable. I’d be glad to prune someone’s yard for free with this thing.

    Matt61


    • Matt,
      Tempering removes hardness to prevent brittle fractures. Hardness comes from heating the steel to a critical temperature and the method of quenching — in a nutshell, the faster the temperature falls, the harder/more brittle the steel becomes, dependent on the type of steel, mainly its carbon content. I don’t think most rifle barrels would be tempered, and I’m not sure they would be intentionally hardened either, although some are work-hardened to a degree by the machining process, I would assume. Annealing is heating to critical temperature and cooling slowly, leaving the steel as soft as possible. I don’t know about modern barrel, but some ML’ing barrels have been annealed in order to make them less brittle. A barrel that bulges or splits is dangerous, but one that fragments because it is brittle is a pipe bomb; that has happened on some modern barrels, esp. a couple of alloys of stainless. Anyway, I don’t think the barrels would be that hard. Possibly and especially if barrel were heated (like a machine gun barrel), it could be cut by a sufficiently hardened/properly tempered knife, just like you can cut steel at an anvil with a chisel. Probably happened once or very few times under special case conditions, then became folklore.


    • On the subject of metal wearing, I’m the proud new possessor of a barong, a sort of machete and short sword favored by tribes in the southern Philippines. So, there I am in the sweaty jungles with this amazing new weapon! Apparently, it achieved notoriety in the Spanish-American war for its ability to cut through rifle barrels. I’ve heard the same about samurai swords in WWII. Is this true? I thought the wisdom is that no blade can cut something harder than itself. I found that out the hard way (in my young and ignorant days) when I ruined my Cold Steel Voyager by using it to try to cut through a steel cable. If rifle tempering is anything close to blade tempering, I don’t see how any sword could cut through a barrel.

      MythBusters took care of that a few years ago… As I recall, the best they obtained was bent barrels after heating them red-hot in a forge and using the pneumatic “super samurai” swinging machine. And lots of shattered blades.


  8. Well, as long as we are cussing and discussing theories, I think that Mack had to show BB how to shoot!

    I get my best groups with my springers directly off my rubber rest that is mounted on my camera tripod. I know that is contrary to what BB normally finds. I suspect that I just don’t keep as steady supporting the gun with my palm or fingers as BB does. When I support off my hand I can see the rifle move up and down as my hear beats. I think the most important things when shooting any springer are to be consistent shot after shot and to hold your position after the trigger is pulled long enough for the pellet to leave the barrel. Most of my misses are pulled off target by moving after the trigger is pulled. Years ago I got into a bad habit of wanting to look up as soon as the trigger is pulled to see where I hit. I have fought that habit for more than 20 years.

    David Enoch


  9. I have 3 guesses; shim/replace breech seal, tighten hinge bolt, or clean and/or lube lockup chisel so it sticks out as far it’s suppose to.

    David H.


  10. Well, count me among the stumped. There are so many things BB has done in the past to improve accuracy, that I don’t think I can pick just one. I’ll side with Matt and out wait him…

    /Dave





  11. BB stopped shooting over his shoulder using a mirror. I saw Felix the cat shoot that way when I was about four years old. I’m not going to guess, too many possibilities. Can’t wait to see what the answer is, I’ve been considering this sight set up for my Bronco.



  12. My guess is it has to be something that has changed betwwen the last time was shot and this time.
    I don’t think changing the sights on a given rifle will need a change of holding method… Or would it?
    BUT it can’t be just the sigths as B.B. said this trick could benefit ALL Bronco…
    Just under a half to go…

    J-F


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